An organic light-emitting diode device, also called an OLED, commonly includes an anode, a cathode, and an organic electroluminescent (EL) unit sandwiched between the anode and the cathode. The organic EL unit typically includes a hole-transporting layer (HTL), a light-emitting layer (LEL), and an electron-transporting layer (ETL). OLEDs are attractive because of their low drive voltage, high luminance, wide viewing-angle, and capability for full color displays and for other applications. Tang et al. described this multilayer OLED in their U.S. Pat. Nos. 4,769,292 and 4,885,211.
OLEDs can emit different colors, such as red, green, blue, or white, depending on the emitting property of its LEL. An OLED with separate red-, green-, and blue-emitting pixels (RGB OLED) can produce a wide range of colors and is also called a full-color OLED. Recently, there is an increasing demand for broadband OLEDs to be incorporated into various applications, such as a solid-state lighting source, color display, or a full color display. By broadband emission, it is meant that an OLED emits sufficiently broadband light throughout the visible spectrum so that such light can be used in conjunction with filters or color change modules to produce displays with at least two different colors or a full color display. In particular, there is a need for broadband-light-emitting OLEDs (or broadband OLEDs) where there is substantial emission in the red, green, and blue portions of the spectrum, i.e., a white-light-emitting OLED (white OLED). The use of white OLEDs with color filters provides a simpler manufacturing process than an OLED having separately patterned red, green, and blue emitters. This can result in higher throughput, increased yield, and cost savings in manufacturing. White OLEDs have been reported, e.g. by Kido et al. in Applied Physics Letters, 64, 815 (1994), J. Shi et al. in U.S. Pat. No. 5,683,823, Sato et al. in JP 07-142169, Deshpande et al. in Applied Physics Letters, 75, 888 (1999), and Tokito, et al. in Applied Physics Letters, 83, 2459 (2003).
However, in contrast to the manufacturing improvements achievable by white OLEDs in comparison to RGB OLEDs, white OLEDs suffer efficiency losses in actual use. This is because each subpixel produces broadband, or white, light, but color filters remove a significant part of the emitted light. For example, in a red subpixel as seen by an observer, an ideal red color filter would remove blue and green light produced by the white emitter, and permit only wavelengths of light corresponding to the perception of red light to pass. A similar loss is seen in green and blue subpixels. The use for color filters, therefore reduces the radiant efficiency to approximately ⅓ of the radiant efficiency of the white OLED. Further, available color filters are often far from ideal, having peak transmissivity significantly less than 100%, with the green and blue color filters often having peak transmissivity below 80%. Finally, to provide a display with a high color gamut, the color filters often need to be narrow bandpass filters and therefore they further reduce the radiant efficiency. In some systems, it is possible for the radiant efficiencies of the resulting red, green, and blue subpixels to have radiant efficiencies on the order of one sixth of the radiant efficiency of the white emitter.
Several methods have been discussed for increasing the efficiency of OLED displays using a white emitter. For example, Miller et al. in U.S. Pat. No. 7,075,242, entitled “Color OLED display system having improved performance” discuss the application of an unfiltered white subpixel to increase the efficiency of such a display. Other disclosures, including Cok et al. in U.S. Pat. No. 7,091,523, entitled “Color OLED device having improved performance” and Miller et al. in U.S. Pat. No. 7,333,080 entitled “Color OLED display with improved power efficiency” have discussed the application of yellow or cyan emitters for improving the efficiency of light emission for a display employing a white emitter.
Other references that describe displays that use multiple primaries include U.S. Pat. No. 7,787,702, US 20070176862; US 20070236135 and US 20080158097.
While these methods improve the efficiency of the resulting display, the improvement is often less than desired for many applications.